Tonometry

Tonometry is a diagnostic test that measures the pressure inside your eye, which is called intraocular pressure (IOP). This measurement can help your doctor determine whether or not you may be at risk of glaucoma. Glaucoma is a serious eye disease in which there’s an increased fluid pressure within your eye. This increased pressure can damage your optic nerve. According to NIH Senior Health, glaucoma is most likely to occur in people over the age of 60 and is a leading cause of blindness within that age group.

In most cases of glaucoma, the fluid that normally bathes and nourishes the eye drains too slowly, causing pressure buildup. Without treatment, that pressure can eventually harm your optic nerve, causing a loss of vision. Because these changes within your eye are often painless, they can progress for years without you noticing.

Since glaucoma can cause eventual blindness if it’s not treated, a tonometry test is critical for detecting eye changes early. If your test results come back abnormal, your eye doctor will begin the treatment process, which can delay the progression of the disease.

Tonometry is an eye test that can detect changes in eye pressure long before you may be aware of them. The most common type of tonometry test is called the "Goldmann applanation tonometry test." For decades, it has been considered the international gold standard for measuring IOP. There are other methods of testing for eye pressure, including an alternative type of tonometry called "pneumotonometry," as well as use of the Tono-Pen.

Pneumotonometry involves applying air pressure to your eye, using an instrument that resembles an air piston. The instrument blows a brief puff of air at your cornea, measuring the pressure in your eye. If it shows abnormal results, your doctor will usually perform other tests to confirm your diagnosis. This test is usually considered less accurate than the Goldmann applanation tonometry test.

The Tono-Pen is a handheld device that’s shaped like a large marker. It provides a digital readout of eye pressure. Your doctor can use it to touch your eye and measure pressure. Though useful in many cases, the Tono-Pen is considered less accurate than the Goldmann tonometer in people with normal corneas.

Your eye doctor may order the Goldman applanation tonometry test if they suspect you may be at risk of glaucoma. If the air puff test or other eye tests indicate a potential problem, they may ask you to go through the tonometry test to confirm or rule out glaucoma.

According to the Mayo Clinic, you may be at higher risk of glaucoma if you’re African-American or Hispanic. You may also be at higher risk if you:

Before the tonometry test, your eye doctor will put numbing eye drops in your eyes so that you don’t feel anything touching them. Once your eye is numb, your doctor may touch a tiny, thin strip of paper that contains orange dye to the surface of your eye to stain it. This helps increase the accuracy of the test.

Your doctor will then put a machine called a "slit-lamp" in front of you. You will rest your chin and forehead on the supports provided. The lamp will then be moved toward your eyes until just the tip of the tonometer probe touches your cornea. By flattening your cornea just a bit, the instrument can detect the pressure in your eye. Your eye doctor will adjust the tension until they get a proper reading.

Because your eyes are numb, you will feel no pain during the procedure. Tonometry is extremely safe. However, there’s a very minimal risk that your cornea could be scratched when the tonometer touches your eye. Even if this happens, however, it will normally heal itself within a few days.

A normal test result means that the pressure in your eye is within the normal range and you don’t have glaucoma or other pressure-related eye problems. According to the Glaucoma Research Foundation, the normal pressure range is 12 to 22 mm Hg. The measurement "mmHg" means "millimeters of mercury," which are the units used to record eye pressure.

If your test comes back with a pressure reading exceeding 20 mm Hg, you may have glaucoma or pre-glaucoma. This isn’t always the case. Your test result can also show high pressure if you’ve had an eye injury, or if you have bleeding in the front of your eye caused by blood vessel problems, inflammation, or other issues.

Your doctor will discuss treatment options with you if they diagnose you with glaucoma or pre-glaucoma.

Medically reviewed by Steve Kim, MD on February 4, 2016 — Written by Colleen M. Story